In the operation of nuclear reactors, the nuclear energy source is in the form of hollow zircaloy tubes filled with enriched uranium, typically referred to as fuel assemblies. When the energy in the fuel assembly has been depleted to a certain level, the assembly is removed from the nuclear reactor. At this time, fuel assemblies, also known as spent nuclear fuel, emit both considerable heat and extremely dangerous neutron and gamma photons (i.e., neutron and gamma radiation). Thus, great caution must be taken when the fuel assemblies are handled, transported, packaged and stored.
After the depleted fuel assemblies are removed from the reactor, they are placed in a canister. Because water is an excellent radiation absorber, the canisters are typically submerged under water in a pool. The pool water also serves to cool the spent fuel assemblies. When fully loaded with spent nuclear fuel, a canister weighs approximately 45 tons. The canisters must then be removed from the pool because it is ideal to store spent nuclear fuel in a dry state.
Removal from the storage pool and transport of the loaded canister to the storage cask is facilitated by a transfer cask. In facilities utilizing transfer casks to transport loaded canisters, an empty canister is placed into the cavity of an open transfer cask. The canister and transfer cask are then submerged in the storage pool. As each assembly of spent nuclear fuel is depleted, it is removed from the reactor and lowered into the storage pool and placed in the submerged canister (which is within the transfer cask). The loaded canister is then fitted with its lid, enclosing the spent nuclear fuel and water from the pool within. The canister and transfer cask are then removed from the pool by a crane and set down in a staging area to prepare the spent nuclear fuel for storage in the “dry state.” Once in the staging area, the water contained in the canister is pumped out of the canister. This is called dewatering. Once dewatered, the spent nuclear fuel is dried using a suitable process such as vacuum drying. Once dry, the canister is back-filled with an inert gas such as helium. The canister is then sealed and the canister and the transfer cask are once again lifted by the plant's crane and transported to an open storage cask. The transfer cask is then placed atop the storage cask and the canister is lowered into the storage cask.
Because a transfer cask must be lifted and handled by a plant's crane (or other equipment), transfer casks are designed to be a smaller and lighter than storage casks. A transfer cask must be small enough to fit in a storage pool and light enough so that, when it is loaded with a canister of spent nuclear fuel, its weight does not exceed the crane's rated weight limit. Additionally, a transfer cask must still perform the important function of providing adequate radiation shielding for both the neutron and gamma radiation emitted by the enclosed spent nuclear fuel. As such, transfer casks are made of a gamma absorbing material such as lead and contain a neutron absorbing material.
However, the allowable weight of a transfer cask is limited by the lifting capacity of the plant's crane (or other lifting equipment). The load handled by the crane includes not only the weight of the transfer cask itself, but also the weight of the transfer cask's payload (i.e., the canister and its contents). A transfer cask must be designed so that the total load handled by the crane during all handling evolutions does not exceed the crane's rated weight limit, which is typically in the range of 100-125 tons.
Because the weight of the transfer cask's payload varies during the different stages of the transport procedure, the permissible weight of the transfer cask is equal to the rated capacity of the plant crane less the weight of the transfer cask's maximum payload at any lifting step. The weight of the transfer cask's payload is at a maximum when the transfer cask and canister are lifted out of the storage pool, at which time the canister is full of spent nuclear fuel and water. Thus, according to prior art methods, it is at this stage that the permissible weight of a transfer cask is calculated. The transfer cask is then constructed using this permissible weight as a design limitation.
Additionally, many nuclear sites have more than one reactor unit and more than one storage pool. Each of the storage pools might have its own crane, and the rated capacity of one crane at one storage pool might be different from the rated capacity of the crane at other storage pools. In nuclear sites with multiple pools and multiple cranes with different rating capacities, it might be desirable to move the depleted fuel assemblies from one pool, with a crane having a lower rating capacity, to another pool having a crane with a higher rating capacity, prior to placing the depleted fuel assemblies into a canister, such as a multi-purpose canister (“MPC”) within a transfer cask. This is because the rated capacity of a crane at one pool might not be able to safely lift a fully loaded transfer cask (with depleted fuel assemblies and canister). Therefore, there is a need for a system and method of transferring the depleted fuel assemblies from one pool, having a crane that cannot safely lift a fully loaded transfer cask, to another pool, having a crane with a rating capacity that can safely lift a fully loaded transfer cask. Since the pools in some of these sites are not interconnected to permit underwater transfer of the depleted fuel assemblies from one pool to another, a transfer canister for inter-unit transfer of depleted fuel assemblies is needed. It is desirable that depleted fuel assembly transfer from one pool to another be accomplished in the minimum amount of time (and hence radiation dose), with multiple assemblies at one time, with minimized upending and downending operations that carry the risk of handling accidents, with minimized (or eliminated) reliance on forced cooling methods that may introduce operation vulnerability to the transfer process, ensuring no risk of a criticality event, and with maximized protection against events such as crane malfunctions.
Thus, a need exists for a method and apparatus for transferring high level radioactive materials from a first submerged environment to a second submerged environment that accomplishes the aforementioned goals.